For purposes of enhancing production from a subterranean formation, a perforating gun is typically lowered down into a wellbore that extends through the formation. A perforating gun typically comprises a plurality of gun sections where each gun section comprises a plurality of radially-oriented shaped charges which are detonated to form perforations in the formation proximate the wellbore. These shaped charges may, for example, be placed at points on a helical spiral that extends along the longitudinal axis of each gun section of the perforating gun.
In wireline guns, there are normally two wires that run the length of the gun string. One of these wires is a live or hot wire which is connected to a positive or negative voltage, and the other wire is a ground wire. These wires connect a source of current and voltage which is located at or near the earth's surface to an electrical detonator in the body of the perforating gun. The electrical detonator is the apparatus that initiates the ballistic train including the detonating cord and subsequently the shaped charges which will perforate the well and permit flow between the reservoir in the formation and the wellbore. The typical initiation system used for a perforating gun string usually also requires ballistics to be run from gun to gun when running several guns in a string.
The presence of the wires not only decreases the amount of space in the gun for shaped charges but also increases the amount of time required to load a gun. If a wire gets pinched or shorts out during a loading or transportation process, the wires must be replaced and reinstalled in the gun, which also increases the time necessary for loading.
Typically, the components of perforating guns except the detonator are assembled at one location and then shipped to a second location where the perforating operation is to be conducted. At that second location, a port is opened in the loading tube and a detonator is installed. The detonator may, for example, be a SECURE™ detonator which is provided by the assignee of the present application, and this detonator includes an addressable switch, a fireset and an initiator. Accordingly, the installation of the detonator assembly at the site where perforating is to take place involves the connection of a number of wires in a very small space. The installation of the detonator also requires the utilization of a safety tube in which the detonator is placed before connection of the detonator to the wiring in the loading tube.
Usually a perforating string will comprise a plurality of perforating guns. The activation of the guns in such a string will normally be from the lowermost gun section to the topmost gun section. If, for example, a perforating string has three gun sections and the middle section is activated first, the activation of the middle section destroys the wires in the middle section, and communication between the earth's surface and the lowermost gun section no longer is possible.
Apparatus comprising a safe system that does not require wires or ballistics to run through each gun in a perforating string would bring a substantial benefit in efficiency, service quality and safety to the perforating operation. Also, a perforating string that does not require the installation of a detonating device at the perforating site would be beneficial both for efficiency and safety. Further, a perforating string in which the gun sections could be activated in any order would be beneficial.
Many of the issues noted above have been addressed by embodiments of the present invention.